1. Field of the Invention
The present invention relates to an ultrasonic flow switch which is operable based on an amount of a fluid which flows in a pipe.
2. Description of Related Art
A flow rate meter is used for accurately measuring a value of a flow rate of a fluid which flows in a pipe. For example, JP-A-2001-356032 discloses an ultrasonic flow rate meter having a sensor. The sensor can measure an amount of a fluid which flows in a pipe using an ultrasonic wave. In the sensor, two sleigh-like blades which project downward are formed. A sheet-like silicone gel is arranged between two blades. The sensor is fixed to an outer peripheral surface of a pipe by being fastened by a fastening band such that bottom sides of two blades are brought into contact with the pipe.
A sheet-shaped silicone gel prevents reflection of an ultrasonic wave between the sensor and the pipe. When the sensor is fixed to the pipe, the sheet-shaped silicone gel is collapsed in conformity with a shape of the pipe. A residual portion of an outer peripheral surface of the pipe is filled with an overflowed portion of the sheet-shaped silicone gel, so that a contact area between the sensor and the pipe is increased. Accordingly, the sensor and the pipe are joined to each other.
JP-A-2001-356032 describes that “it is sufficient to fasten a fastening band to an extent that bottom sides of sleigh-like blades are always brought into contact with a pipe so that uniformity and reproducibility of installation of the sensor are enhanced”. Further, JP-A-2001-356032 describes that “the sensor can be stably fixed to a pipe, regardless of a position where the sensor is mounted, such as an upper side or a lower side of a horizontal part, a vertical part or an inclined part of the pipe, and can be stably fixed to pipes with almost all pipe diameters except pipe diameters equal to or less than a distance between the two sleigh-like blades”.
However, in an actual installation of the sensor, when the sensor is fixed to the pipe having a diameter larger than the distance between two sleigh-like blades, the degree that a sheet-shaped silicone gel is collapsed differs depending on a size of the pipe.
It is not always possible to acquire uniformity when joining the sensor and the pipe and hence, it is difficult to measure a flow rate with high reproducibility. Further, when the sensor part is fastened excessively strongly by the band to bring the bottom sides of sleigh-like blades into contact with the pipe, a sheet-shaped silicone gel may be damaged.
On the other hand, as in the case of controlling an operation state of a facility in a plant, there may be a case where an accurate value of a flow rate of a fluid which flows in a pipe is unnecessary and it is sufficient to detect whether or not the fluid flows in the pipe at a flow rate of a fixed value or more. In such a case, a flow rate switch which outputs an ON/OFF signal can be used instead of a flow rate meter. The flow switch is mounted on pipes having various outer diameters in a plant, for example. Accordingly, it is desirable that the flow switch can be stably mounted on pipes having various sizes without impairing uniformity of performance of the flow switch.